After only a somewhat recent introduction, optical fiber has had a meteoric rise as the predominant means of transmission media in voice and data communications. Optical fiber is manufactured by drawing the fiber from a preform which is made by any of several well known processes. Afterwards, or as part of a tandem process, the drawn fiber is coated, cured, measured and taken up, desirably in an automatic takeup apparatus, on a spool to provide a package. Typically, an optical fiber has a diameter on the order of 125 microns, for example, and is covered with a coating material which increases the outer diameter of the coated fiber to about 250 microns, for example. Methods and apparatus for taking up optical fiber are disclosed and claimed in U.S. Pat. No. 4,798,346 which issued on Jan. 17, 1989 in the names of D. L. Myers and J. W. Wright.
An optical fiber package is used in operations such as ribboning, cabling, and rewinding and is used to ship optical fiber to other companies which further process the fiber. The optical fiber typically is used in voice and data communications systems, both commercial and military. For example, the package may be used in weapons systems in which it is used for guidance and for data communications. Such uses include communication lines between aircraft, between an aircraft and a ship, and between a projectile, such as a missile, and a control station at a launch site, for example. Optical fiber provides the advantages of increased data bandwidth, reduced weight and greater range than wire-guided systems of the prior art.
There are, however, certain disadvantages, not present in other forms of communication, in using optical fiber. Optical fiber is less robust than metallic conductors, rendering it subject to breakage. Aside from breakage, optical fiber communication performance may be degraded by microbends in the fiber which are generated by bending or by other stresses to which the fiber may be subjected. Such damage to an optical fiber not only reduces the long-term durability of the fiber, but also causes losses in the strength and in the content of the optical signal. Likewise, physical or optical integrity may be affected adversely by any sharp bends which are experienced as the fiber pays out from its packaged configuration.
A typical optical fiber application in a weapons systems involves the packaging of a continuous length of optical fiber on a bobbin which is positioned inside a vehicle. Such a vehicle commonly is referred to as a tethered vehicle. One end of the fiber is attached to operational devices in the vehicle, whereas the other end of the fiber is connected to a control or communications station at a launch site. During and after launch, two-way communication with the vehicle is conducted.
In order to use such an arrangement, there must be provided a reliable and compact package of the optical fiber which may be disposed within the vehicle and which will permit reliable deployment of the optical fiber during the flight of the vehicle. The use of metallic conductors for guidance or control of launched vehicles is known. Although the art teaches the use of bobbins on which a metallic conductor is wound, the fragility of optical fiber requires specialized treatment that facilitates the unwinding of the optical fiber from its bobbin at a relatively high rate of speed.
One problem is that extremely long lengths of fiber may be required. These may be obtained by splicing a plurality of lengths. Typically, the original coating material is removed from an end portion of each of the two coated fibers to be spliced. The removal is such that the end of the coating material remaining on the end portion is a surface which is normal to the axis of the fiber. The recoating material contacts the adjacent originally coated portions of the spliced fibers along those normal surfaces and along overlapping portions of the outer surface of the original coating material adjacent to the normal surfaces. The coating material is then cured to yield a recoated splice section with a transverse cross section which is larger than that of the optical fiber having the original coating material thereon.
For tethered vehicles, the winding of the optical fiber on the payoff device must be accomplished in a precise manner. Otherwise, payoff could be disrupted. It has been found that if the cross section of the recoated portion transverse of the longitudinal axis of the optical fiber is not the same as that of the optical fiber as originally coated, the winding pattern on the payoff device in all likelihood is not uniform. This will cause problems in fiber payoff following the launch of the tethered vehicle.
Another problem in the optical fiber guidance of tethered vehicles relates to the successful unwinding of the fiber from a bobbin as the bobbin is propelled along with the vehicle. In optical fiber packages for use in tethered vehicles, as many as at least thirty layers of optical fiber are wound on a guiding structure. The leading end of the optical fiber is connected to a guidance system for controlling the path of travel of the vehicle. It becomes important for the optical fiber to be payed off from the bobbin without the occurrence of snags, or tight bends, otherwise the fiber may break or the signal may be attenuated and the control system rendered inoperable. Contributing to the successful payout of the optical fiber is a precision wound package. Not only must the convolutions be wound with precision, they also must remain in place as wound during handling and during deployment. In other words, the optical fiber package must be a highly stable one.
An adhesive material between the optical fiber turns on the bobbin must function to hold the package together, forming a stable structure which is resistant to environmental extremes, shock and vibration. On the other hand, payout must occur easily without the necessity of high pulling forces to remove each convolution of fiber from the package. Desirably, the adhesive material which is used to hold together the convolutions must have a minimal impact on the optical performance of the wound fiber, and yet it must allow the optical fiber to be payed out with a controlled force at the peel-off point as each successive outermost turn is unwound at high speed. These requirements of stability and ease of payout present somewhat conflicting requirements for the adhesive system.
During storage and transport of the bobbin, mechanical stability is most important as the adhesive adds integrity to the wound package, thereby maintaining the package in a ready condition for deployment. During deployment, both mechanical and optical effects are significant. The adhesive system must provide tack which is sufficiently low to permit a helical pattern of payout at potentially high speeds, possibly approaching or exceeding Mach 1. Excessive tack threatens fiber integrity by forming an extreme bend at the peel-off point. On the other hand, not enough tack may result in failure through dynamic instability on the bobbin. With respect to optical performance, optical attenuation at the peel-off point may occur through localized macrobending, degrading the integrity of data and video transmission. Also, microbending in the layers of undeployed fiber on the bobbin during deployment can affect adversely optical performance.
It has been found that the adhesive material can contribute significantly to attenuation increases, especially at lower temperatures. Total permissible losses must be maintained within system limits set by the transmission opto-electronics over a specified operational temperature range, for example from -25.degree. to 60.degree. C. Typical initial optical fiber losses may comprise a portion of the attenuation budget, emphasizing the need to keep additional attenuation-inducing effects low for increased range.
The primary mechanism for adhesive-induced attenuation is believed to be the quality of the surface of the adhesive material which has been applied to the fiber layers. A rough surface, for example, causes microbends on the optical fiber in engagement with the rough surface because of the pressures developed during winding. These microbends are intensified as the adhesive material and the optical fiber coating stiffen with decreasing temperature.
Current techniques for providing a guided vehicle which includes such a stable package include the coating of each layer of fiber convolutions after they have been wound on the bobbin. In the prior art, at least one system includes a spraying apparatus. The apparatus is used to apply a liquid adhesive material to the optical fiber convolutions as they are wound on a bobbin. In the development of optical fiber use for tethered vehicles, a polychloroprene rubber cement adhesive material has been used. Problems have been encountered with that material insofar as aging characteristics and attenuation performance over a wide temperature are concerned.
What is needed and what has not been available in the prior art is a guided vehicle system which includes a bobbin of precision wound optical fiber in which the convolutions of fiber are held together by an adhesive material. The adhesive material should be such that it stabilizes the package yet permits payout at relatively high speeds. And, importantly, the surface portion of each convolution which is coated with the adhesive material must not be too rough, thereby avoiding microbending of the optical fiber.